Among the petrophysical properties of subterranean geologic formations that serve as hydrocarbon (crude oil and/or natural gas) reservoirs, porosity, water saturation, and permeability are the most important properties. Porosity and water saturation determine the volume of hydrocarbons that might be present in a given formation since the hydrocarbon content equals the porosity times one minus water saturation. Permeability determines the rate at which hydrocarbons can be produced from the formation rock. These three properties are all influenced by the presence of clay minerals in the formation.
Clay in a formation can cause reductions in porosity and permeability as well as complicate the determination of water saturation of the formation. Because of the influence of clay on these petrophysical properties, the determination of clay content of hydrocarbon-bearing subterranean formations is a critical step in their evaluation. Clay content is normally determined using wireline logs that respond to the high level of radioactivity associated with many clay minerals or to their effects on neutron and density logs. The interpretation of these logs provides a means of determining relative amounts of clay, but measurements on a well core taken from the formation are required to obtain quantitative data for calibration of such logs. Logs in general, and wireline logs in particular, are well known in the art. For example, see "Essentials of Modern Open-Hole Log Interpretation" by J. T. Dewan, PennWell Publishing Co., Tulsa, Okla., 1983.
Clay can exist in sedimentary formation rocks in a wide variety of forms and types. Clay content can be estimated by visual observation of a well core, or a sample therefrom, that has been taken from a formation, but such a determination is purely qualitative and not suitable for the calibration of wireline log models. Clay present as dispersed minerals in sandstone or carbonate rock is particularly difficult to determine by the naked eye. Quantitative determination of the clay content and clay mineralogy can be carried out using analytical techniques such as X-ray diffraction, mineral grain counts of thin, translucent sections of the well core, i.e., thin sections, and the like. These techniques are time-consuming, expensive, represent only a small formation volume, and are destructive of the well core sample. Because of these limitations, these techniques are often not obtained with sufficient data points for log model calibration.
Clay minerals have a large surface area relative to other rock-forming minerals. The surface properties of clays are such that they retain a few monolayers of bound water even after the movable water and other liquids have been removed from the pores and capillaries of the well core sample. Some clay minerals such as smectite contain water bound up as part of the mineral lattice itself. So, the retention of bound water by clays is much more pronounced than with that of non-clay minerals normally found in hydrocarbon-bearing formations. The different types of clays retain varying amounts of bound water after the movable water has been removed from the well core. Thus, even after a well core has been dried there will continue to be small amounts of bound water present in association with the clay minerals in the well core, and the amount of such bound water will vary with the type of clay mineral to which it is bound. Therefore, the measurement of the relative water content along a well core or portion thereof is an indirect indicator of the clay content thereof and even of the types of clay minerals therein.
The dielectric constant of rocks is influenced by their water content so that dielectric constants can be made to provide an indirect measurement of the clay content of a well core or portion thereof. Since the water content of various clays varies with the clay mineralogy, dielectric constants can be used as an indirect indicator of clay types present as well. For example, a clay such as kaolinite which has a relatively low surface area for a clay mineral has relatively small amounts of bound water for a clay, whereas smectite, which has a high surface area, retains larger amounts of bound water. Therefore, after the removal of all or essentially all of the movable water from the well core, and at the same ambient relative humidity around the well core, formation rock in the well core with a higher clay content will have a higher bound water content than formation rock in the well core that has a lower clay content, and rocks with a low clay content will have more bound water than non-clay rocks. Dielectric constants are well known in the art. For example, see "Well Logging for Physical Properties" by J. R. Hearst and P. H. Nelson, McGraw-Hill Book Co., New York, 1985.